Add Hot Spring Connected Spa integration

Unofficial cloud integration for Watkins/Hot Spring "Connected Spa" hot tubs.
Reverse-engineered from the official Android app.

- climate entity (setpoint + current water temp) via heater/control
- sensors: water temperature, salt output level, salt cartridge
- REST (JWT) login + spa discovery; mTLS fetch of rotating HiveMQ broker creds;
  MQTT subscribe (telemetry) / publish (control)
- config flow prompts for account email + password (stored encrypted, never in repo)
- no secrets committed: vendor mTLS client cert is loaded from host PEM files
  (see README); .gitignore blocks pem/p12/der/key/env

Fixes bug-41mqxddz7zeh

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F6B8b3iYNv6QUftK2FDfYb

.gitignore

@@ -0,0 +1,16 @@
+# Never commit credentials or vendor certificate/key material.
+*.pem
+*.p12
+*.pfx
+*.der
+*.key
+*.crt
+certs/
+secrets*
+.env
+*.env
+
+# Python
+__pycache__/
+*.pyc
+.venv/